Borrowed receive

Receiving tokens normally creates a new UTXO, and every UTXO needs some KAS. Borrowed receive (KCC-20 §5) lets a sender use one of the recipient’s existing token UTXOs as the destination instead. The UTXO is “borrowed” for one transaction and recreated with a higher amount, without the recipient’s signature.

The rules

A borrowed receive is allowed only on the transfer leader’s path, so at most one state per family per transaction is borrowed. The borrowed successor is the first output of the family, next_states[0]. It must:

• increase the token amount;
• keep owner, owner_scheme, borrow_scheme and extension_commitment unchanged;
• use the expected next borrow_guard;
• carry at least the KAS value of the borrowed input.

A borrower can never move tokens to another owner, lower the amount, or reduce the KAS in the UTXO. It does change the UTXO’s outpoint.

Borrow schemes

Byte	Scheme	borrow_guard holds	Who may borrow
0x00	disabled/v1	Unused	Nobody
0x01	amount-threshold/v1	A threshold in the first 8 bytes	Anyone who adds more than the threshold
0x02	schnorr-signature/v1	A Schnorr public key	The holder of that key
0x03	hash-chain/v1	The current hash-chain commitment	Whoever holds the next released link, once per link

The cost

Because borrowing changes the outpoint, it invalidates any unconfirmed transaction that spends the old one, and the owner’s wallet must find the successor before spending. Repeated borrowing can deny an owner a stable outpoint.

Where kUSDT disables it

State	Borrow scheme	Why
Every freshly minted kUSDT state	0x00, guard all zero	BridgeMint enforces it on every recipient
A redemption burn state	0x00 (required by the specification)	Otherwise anyone could borrow it: add one base unit and recreate it at a new outpoint, which would look like a new burn of the whole amount
The KasPool kUSDT reserve	0x00	The pool forces it on every reserve successor, so its outpoint changes only through the pool

A user may turn borrowing on for their own states later, with a normal owner-approved transfer.